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stock binutils-2.8
1997-09-24 19:39:15 +04:00
This is Info file ld.info, produced by Makeinfo-1.64 from the input
file ./ld.texinfo.
START-INFO-DIR-ENTRY
* Ld: (ld). The GNU linker.
END-INFO-DIR-ENTRY
This file documents the GNU linker LD.
Copyright (C) 1991, 92, 93, 94, 95, 96, 1997 Free Software
Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the entire resulting derived work is distributed under the terms
of a permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.

File: ld.info, Node: Top, Next: Overview, Up: (dir)
Using ld
********
This file documents the GNU linker ld.
* Menu:
* Overview:: Overview
* Invocation:: Invocation
* Commands:: Command Language
* Machine Dependent:: Machine Dependent Features
* BFD:: BFD
* Reporting Bugs:: Reporting Bugs
* MRI:: MRI Compatible Script Files
* Index:: Index

File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top
Overview
********
`ld' combines a number of object and archive files, relocates their
data and ties up symbol references. Usually the last step in compiling
a program is to run `ld'.
`ld' accepts Linker Command Language files written in a superset of
AT&T's Link Editor Command Language syntax, to provide explicit and
total control over the linking process.
This version of `ld' uses the general purpose BFD libraries to
operate on object files. This allows `ld' to read, combine, and write
object files in many different formats--for example, COFF or `a.out'.
Different formats may be linked together to produce any available kind
of object file. *Note BFD::, for more information.
Aside from its flexibility, the GNU linker is more helpful than other
linkers in providing diagnostic information. Many linkers abandon
execution immediately upon encountering an error; whenever possible,
`ld' continues executing, allowing you to identify other errors (or, in
some cases, to get an output file in spite of the error).

File: ld.info, Node: Invocation, Next: Commands, Prev: Overview, Up: Top
Invocation
**********
The GNU linker `ld' is meant to cover a broad range of situations,
and to be as compatible as possible with other linkers. As a result,
you have many choices to control its behavior.
* Menu:
* Options:: Command Line Options
* Environment:: Environment Variables

File: ld.info, Node: Options, Next: Environment, Up: Invocation
Command Line Options
====================
The linker supports a plethora of command-line options, but in actual
practice few of them are used in any particular context. For instance,
a frequent use of `ld' is to link standard Unix object files on a
standard, supported Unix system. On such a system, to link a file
`hello.o':
ld -o OUTPUT /lib/crt0.o hello.o -lc
This tells `ld' to produce a file called OUTPUT as the result of
linking the file `/lib/crt0.o' with `hello.o' and the library `libc.a',
which will come from the standard search directories. (See the
discussion of the `-l' option below.)
The command-line options to `ld' may be specified in any order, and
may be repeated at will. Repeating most options with a different
argument will either have no further effect, or override prior
occurrences (those further to the left on the command line) of that
option. Options which may be meaningfully specified more than once are
noted in the descriptions below.
Non-option arguments are objects files which are to be linked
together. They may follow, precede, or be mixed in with command-line
options, except that an object file argument may not be placed between
an option and its argument.
Usually the linker is invoked with at least one object file, but you
can specify other forms of binary input files using `-l', `-R', and the
script command language. If *no* binary input files at all are
specified, the linker does not produce any output, and issues the
message `No input files'.
If the linker can not recognize the format of an object file, it will
assume that it is a linker script. A script specified in this way
augments the main linker script used for the link (either the default
linker script or the one specified by using `-T'). This feature
permits the linker to link against a file which appears to be an object
or an archive, but actually merely defines some symbol values, or uses
`INPUT' or `GROUP' to load other objects. Note that specifying a
script in this way should only be used to augment the main linker
script; if you want to use some command that logically can only appear
once, such as the `SECTIONS' or `MEMORY' command, you must replace the
default linker script using the `-T' option. *Note Commands::.
For options whose names are a single letter, option arguments must
either follow the option letter without intervening whitespace, or be
given as separate arguments immediately following the option that
requires them.
For options whose names are multiple letters, either one dash or two
can precede the option name; for example, `--oformat' and `--oformat'
are equivalent. Arguments to multiple-letter options must either be
separated from the option name by an equals sign, or be given as
separate arguments immediately following the option that requires them.
For example, `--oformat srec' and `--oformat=srec' are equivalent.
Unique abbreviations of the names of multiple-letter options are
accepted.
`-aKEYWORD'
This option is supported for HP/UX compatibility. The KEYWORD
argument must be one of the strings `archive', `shared', or
`default'. `-aarchive' is functionally equivalent to `-Bstatic',
and the other two keywords are functionally equivalent to
`-Bdynamic'. This option may be used any number of times.
`-AARCHITECTURE'
`--architecture=ARCHITECTURE'
In the current release of `ld', this option is useful only for the
Intel 960 family of architectures. In that `ld' configuration, the
ARCHITECTURE argument identifies the particular architecture in
the 960 family, enabling some safeguards and modifying the
archive-library search path. *Note `ld' and the Intel 960 family:
i960, for details.
Future releases of `ld' may support similar functionality for
other architecture families.
`-b INPUT-FORMAT'
`--format=INPUT-FORMAT'
`ld' may be configured to support more than one kind of object
file. If your `ld' is configured this way, you can use the `-b'
option to specify the binary format for input object files that
follow this option on the command line. Even when `ld' is
configured to support alternative object formats, you don't
usually need to specify this, as `ld' should be configured to
expect as a default input format the most usual format on each
machine. INPUT-FORMAT is a text string, the name of a particular
format supported by the BFD libraries. (You can list the
available binary formats with `objdump -i'.) *Note BFD::.
You may want to use this option if you are linking files with an
unusual binary format. You can also use `-b' to switch formats
explicitly (when linking object files of different formats), by
including `-b INPUT-FORMAT' before each group of object files in a
particular format.
The default format is taken from the environment variable
`GNUTARGET'. *Note Environment::. You can also define the input
format from a script, using the command `TARGET'; see *Note Option
Commands::.
`-c MRI-COMMANDFILE'
`--mri-script=MRI-COMMANDFILE'
For compatibility with linkers produced by MRI, `ld' accepts script
files written in an alternate, restricted command language,
described in *Note MRI Compatible Script Files: MRI. Introduce
MRI script files with the option `-c'; use the `-T' option to run
linker scripts written in the general-purpose `ld' scripting
language. If MRI-CMDFILE does not exist, `ld' looks for it in the
directories specified by any `-L' options.
`-d'
`-dc'
`-dp'
These three options are equivalent; multiple forms are supported
for compatibility with other linkers. They assign space to common
symbols even if a relocatable output file is specified (with
`-r'). The script command `FORCE_COMMON_ALLOCATION' has the same
effect. *Note Option Commands::.
`-e ENTRY'
`--entry=ENTRY'
Use ENTRY as the explicit symbol for beginning execution of your
program, rather than the default entry point. *Note Entry Point::,
for a discussion of defaults and other ways of specifying the
entry point.
`-E'
`--export-dynamic'
When creating a dynamically linked executable, add all symbols to
the dynamic symbol table. Normally, the dynamic symbol table
contains only symbols which are used by a dynamic object. This
option is needed for some uses of `dlopen'.
`-f'
`--auxiliary NAME'
When creating an ELF shared object, set the internal DT_AUXILIARY
field to the specified name. This tells the dynamic linker that
the symbol table of the shared object should be used as an
auxiliary filter on the symbol table of the shared object NAME.
If you later link a program against this filter object, then, when
you run the program, the dynamic linker will see the DT_AUXILIARY
field. If the dynamic linker resolves any symbols from the filter
object, it will first check whether there is a definition in the
shared object NAME. If there is one, it will be used instead of
the definition in the filter object. The shared object NAME need
not exist. Thus the shared object NAME may be used to provide an
alternative implementation of certain functions, perhaps for
debugging or for machine specific performance.
This option may be specified more than once. The DT_AUXILIARY
entries will be created in the order in which they appear on the
command line.
`-F NAME'
`--filter NAME'
When creating an ELF shared object, set the internal DT_FILTER
field to the specified name. This tells the dynamic linker that
the symbol table of the shared object which is being created
should be used as a filter on the symbol table of the shared
object NAME.
If you later link a program against this filter object, then, when
you run the program, the dynamic linker will see the DT_FILTER
field. The dynamic linker will resolve symbols according to the
symbol table of the filter object as usual, but it will actually
link to the definitions found in the shared object NAME. Thus the
filter object can be used to select a subset of the symbols
provided by the object NAME.
Some older linkers used the `-F' option throughout a compilation
toolchain for specifying object-file format for both input and
output object files. The GNU linker uses other mechanisms for this
purpose: the `-b', `--format', `--oformat' options, the `TARGET'
command in linker scripts, and the `GNUTARGET' environment
variable. The GNU linker will ignore the `-F' option when not
creating an ELF shared object.
`--force-exe-suffix'
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a
`.exe' or `.dll' suffix, this option forces the linker to copy the
output file to one of the same name with a `.exe' suffix. This
option is useful when using unmodified Unix makefiles on a
Microsoft Windows host, since some versions of Windows won't run
an image unless it ends in a `.exe' suffix.
`-g'
Ignored. Provided for compatibility with other tools.
`-GVALUE'
`--gpsize=VALUE'
Set the maximum size of objects to be optimized using the GP
register to SIZE. This is only meaningful for object file formats
such as MIPS ECOFF which supports putting large and small objects
into different sections. This is ignored for other object file
formats.
`-hNAME'
`-soname=NAME'
When creating an ELF shared object, set the internal DT_SONAME
field to the specified name. When an executable is linked with a
shared object which has a DT_SONAME field, then when the
executable is run the dynamic linker will attempt to load the
shared object specified by the DT_SONAME field rather than the
using the file name given to the linker.
`-i'
Perform an incremental link (same as option `-r').
`-lARCHIVE'
`--library=ARCHIVE'
Add archive file ARCHIVE to the list of files to link. This
option may be used any number of times. `ld' will search its
path-list for occurrences of `libARCHIVE.a' for every ARCHIVE
specified.
On systems which support shared libraries, `ld' may also search for
libraries with extensions other than `.a'. Specifically, on ELF
and SunOS systems, `ld' will search a directory for a library with
an extension of `.so' before searching for one with an extension of
`.a'. By convention, a `.so' extension indicates a shared library.
The linker will search an archive only once, at the location where
it is specified on the command line. If the archive defines a
symbol which was undefined in some object which appeared before
the archive on the command line, the linker will include the
appropriate file(s) from the archive. However, an undefined
symbol in an object appearing later on the command line will not
cause the linker to search the archive again.
See the `-(' option for a way to force the linker to search
archives multiple times.
You may list the same archive multiple times on the command line.
This type of archive searching is standard for Unix linkers.
However, if you are using `ld' on AIX, note that it is different
from the behaviour of the AIX linker.
`-LSEARCHDIR'
`--library-path=SEARCHDIR'
Add path SEARCHDIR to the list of paths that `ld' will search for
archive libraries and `ld' control scripts. You may use this
option any number of times. The directories are searched in the
order in which they are specified on the command line.
Directories specified on the command line are searched before the
default directories. All `-L' options apply to all `-l' options,
regardless of the order in which the options appear.
The default set of paths searched (without being specified with
`-L') depends on which emulation mode `ld' is using, and in some
cases also on how it was configured. *Note Environment::.
The paths can also be specified in a link script with the
`SEARCH_DIR' command. Directories specified this way are searched
at the point in which the linker script appears in the command
line.
`-mEMULATION'
Emulate the EMULATION linker. You can list the available
emulations with the `--verbose' or `-V' options. The default
depends on how your `ld' was configured.
`-M'
`--print-map'
Print (to the standard output) a link map--diagnostic information
about where symbols are mapped by `ld', and information on global
common storage allocation.
`-n'
`--nmagic'
Set the text segment to be read only, and mark the output as
`NMAGIC' if possible.
`-N'
`--omagic'
Set the text and data sections to be readable and writable. Also,
do not page-align the data segment. If the output format supports
Unix style magic numbers, mark the output as `OMAGIC'.
`-o OUTPUT'
`--output=OUTPUT'
Use OUTPUT as the name for the program produced by `ld'; if this
option is not specified, the name `a.out' is used by default. The
script command `OUTPUT' can also specify the output file name.
`-r'
`--relocateable'
Generate relocatable output--i.e., generate an output file that
can in turn serve as input to `ld'. This is often called "partial
linking". As a side effect, in environments that support standard
Unix magic numbers, this option also sets the output file's magic
number to `OMAGIC'. If this option is not specified, an absolute
file is produced. When linking C++ programs, this option *will
not* resolve references to constructors; to do that, use `-Ur'.
This option does the same thing as `-i'.
`-R FILENAME'
`--just-symbols=FILENAME'
Read symbol names and their addresses from FILENAME, but do not
relocate it or include it in the output. This allows your output
file to refer symbolically to absolute locations of memory defined
in other programs. You may use this option more than once.
For compatibility with other ELF linkers, if the `-R' option is
followed by a directory name, rather than a file name, it is
treated as the `-rpath' option.
`-s'
`--strip-all'
Omit all symbol information from the output file.
`-S'
`--strip-debug'
Omit debugger symbol information (but not all symbols) from the
output file.
`-t'
`--trace'
Print the names of the input files as `ld' processes them.
`-T COMMANDFILE'
`--script=COMMANDFILE'
Read link commands from the file COMMANDFILE. These commands
replace `ld''s default link script (rather than adding to it), so
COMMANDFILE must specify everything necessary to describe the
target format. You must use this option if you want to use a
command which can only appear once in a linker script, such as the
`SECTIONS' or `MEMORY' command. *Note Commands::. If COMMANDFILE
does not exist, `ld' looks for it in the directories specified by
any preceding `-L' options. Multiple `-T' options accumulate.
`-u SYMBOL'
`--undefined=SYMBOL'
Force SYMBOL to be entered in the output file as an undefined
symbol. Doing this may, for example, trigger linking of
additional modules from standard libraries. `-u' may be repeated
with different option arguments to enter additional undefined
symbols.
`-v'
`--version'
`-V'
Display the version number for `ld'. The `-V' option also lists
the supported emulations.
`-x'
`--discard-all'
Delete all local symbols.
`-X'
`--discard-locals'
Delete all temporary local symbols. For most targets, this is all
local symbols whose names begin with `L'.
`-y SYMBOL'
`--trace-symbol=SYMBOL'
Print the name of each linked file in which SYMBOL appears. This
option may be given any number of times. On many systems it is
necessary to prepend an underscore.
This option is useful when you have an undefined symbol in your
link but don't know where the reference is coming from.
`-Y PATH'
Add PATH to the default library search path. This option exists
for Solaris compatibility.
`-z KEYWORD'
This option is ignored for Solaris compatibility.
`-( ARCHIVES -)'
`--start-group ARCHIVES --end-group'
The ARCHIVES should be a list of archive files. They may be
either explicit file names, or `-l' options.
The specified archives are searched repeatedly until no new
undefined references are created. Normally, an archive is
searched only once in the order that it is specified on the
command line. If a symbol in that archive is needed to resolve an
undefined symbol referred to by an object in an archive that
appears later on the command line, the linker would not be able to
resolve that reference. By grouping the archives, they all be
searched repeatedly until all possible references are resolved.
Using this option has a significant performance cost. It is best
to use it only when there are unavoidable circular references
between two or more archives.
`-assert KEYWORD'
This option is ignored for SunOS compatibility.
`-Bdynamic'
`-dy'
`-call_shared'
Link against dynamic libraries. This is only meaningful on
platforms for which shared libraries are supported. This option
is normally the default on such platforms. The different variants
of this option are for compatibility with various systems. You
may use this option multiple times on the command line: it affects
library searching for `-l' options which follow it.
`-Bstatic'
`-dn'
`-non_shared'
`-static'
Do not link against shared libraries. This is only meaningful on
platforms for which shared libraries are supported. The different
variants of this option are for compatibility with various
systems. You may use this option multiple times on the command
line: it affects library searching for `-l' options which follow
it.
`-Bsymbolic'
When creating a shared library, bind references to global symbols
to the definition within the shared library, if any. Normally, it
is possible for a program linked against a shared library to
override the definition within the shared library. This option is
only meaningful on ELF platforms which support shared libraries.
`--cref'
Output a cross reference table. If a linker map file is being
generated, the cross reference table is printed to the map file.
Otherwise, it is printed on the standard output.
The format of the table is intentionally simple, so that it may be
easily processed by a script if necessary. The symbols are
printed out, sorted by name. For each symbol, a list of file
names is given. If the symbol is defined, the first file listed
is the location of the definition. The remaining files contain
references to the symbol.
`--defsym SYMBOL=EXPRESSION'
Create a global symbol in the output file, containing the absolute
address given by EXPRESSION. You may use this option as many
times as necessary to define multiple symbols in the command line.
A limited form of arithmetic is supported for the EXPRESSION in
this context: you may give a hexadecimal constant or the name of
an existing symbol, or use `+' and `-' to add or subtract
hexadecimal constants or symbols. If you need more elaborate
expressions, consider using the linker command language from a
script (*note Assignment: Symbol Definitions: Assignment.).
*Note:* there should be no white space between SYMBOL, the equals
sign ("="), and EXPRESSION.
`--dynamic-linker FILE'
Set the name of the dynamic linker. This is only meaningful when
generating dynamically linked ELF executables. The default dynamic
linker is normally correct; don't use this unless you know what
you are doing.
`-EB'
Link big-endian objects. This affects the default output format.
`-EL'
Link little-endian objects. This affects the default output
format.
`--embedded-relocs'
This option is only meaningful when linking MIPS embedded PIC code,
generated by the -membedded-pic option to the GNU compiler and
assembler. It causes the linker to create a table which may be
used at runtime to relocate any data which was statically
initialized to pointer values. See the code in testsuite/ld-empic
for details.
`--help'
Print a summary of the command-line options on the standard output
and exit.
`-Map MAPFILE'
Print to the file MAPFILE a link map--diagnostic information about
where symbols are mapped by `ld', and information on global common
storage allocation.
`--no-keep-memory'
`ld' normally optimizes for speed over memory usage by caching the
symbol tables of input files in memory. This option tells `ld' to
instead optimize for memory usage, by rereading the symbol tables
as necessary. This may be required if `ld' runs out of memory
space while linking a large executable.
`--no-whole-archive'
Turn off the effect of the `--whole-archive' option for subsequent
archive files.
`--noinhibit-exec'
Retain the executable output file whenever it is still usable.
Normally, the linker will not produce an output file if it
encounters errors during the link process; it exits without
writing an output file when it issues any error whatsoever.
`--oformat OUTPUT-FORMAT'
`ld' may be configured to support more than one kind of object
file. If your `ld' is configured this way, you can use the
`--oformat' option to specify the binary format for the output
object file. Even when `ld' is configured to support alternative
object formats, you don't usually need to specify this, as `ld'
should be configured to produce as a default output format the most
usual format on each machine. OUTPUT-FORMAT is a text string, the
name of a particular format supported by the BFD libraries. (You
can list the available binary formats with `objdump -i'.) The
script command `OUTPUT_FORMAT' can also specify the output format,
but this option overrides it. *Note BFD::.
`-qmagic'
This option is ignored for Linux compatibility.
`-Qy'
This option is ignored for SVR4 compatibility.
`--relax'
An option with machine dependent effects. This option is only
supported on a few targets. *Note `ld' and the H8/300: H8/300.
*Note `ld' and the Intel 960 family: i960.
On some platforms, the `--relax' option performs global
optimizations that become possible when the linker resolves
addressing in the program, such as relaxing address modes and
synthesizing new instructions in the output object file.
On platforms where this is not supported, `--relax' is accepted,
but ignored.
`--retain-symbols-file FILENAME'
Retain *only* the symbols listed in the file FILENAME, discarding
all others. FILENAME is simply a flat file, with one symbol name
per line. This option is especially useful in environments (such
as VxWorks) where a large global symbol table is accumulated
gradually, to conserve run-time memory.
`--retain-symbols-file' does *not* discard undefined symbols, or
symbols needed for relocations.
You may only specify `--retain-symbols-file' once in the command
line. It overrides `-s' and `-S'.
`-rpath DIR'
Add a directory to the runtime library search path. This is used
when linking an ELF executable with shared objects. All `-rpath'
arguments are concatenated and passed to the runtime linker, which
uses them to locate shared objects at runtime. The `-rpath'
option is also used when locating shared objects which are needed
by shared objects explicitly included in the link; see the
description of the `-rpath-link' option. If `-rpath' is not used
when linking an ELF executable, the contents of the environment
variable `LD_RUN_PATH' will be used if it is defined.
The `-rpath' option may also be used on SunOS. By default, on
SunOS, the linker will form a runtime search patch out of all the
`-L' options it is given. If a `-rpath' option is used, the
runtime search path will be formed exclusively using the `-rpath'
options, ignoring the `-L' options. This can be useful when using
gcc, which adds many `-L' options which may be on NFS mounted
filesystems.
For compatibility with other ELF linkers, if the `-R' option is
followed by a directory name, rather than a file name, it is
treated as the `-rpath' option.
`-rpath-link DIR'
When using ELF or SunOS, one shared library may require another.
This happens when an `ld -shared' link includes a shared library
as one of the input files.
When the linker encounters such a dependency when doing a
non-shared, non-relocateable link, it will automatically try to
locate the required shared library and include it in the link, if
it is not included explicitly. In such a case, the `-rpath-link'
option specifies the first set of directories to search. The
`-rpath-link' option may specify a sequence of directory names
either by specifying a list of names separated by colons, or by
appearing multiple times.
The linker uses the following search paths to locate required
shared libraries.
1. Any directories specified by `-rpath-link' options.
2. Any directories specified by `-rpath' options. The difference
between `-rpath' and `-rpath-link' is that directories
specified by `-rpath' options are included in the executable
and used at runtime, whereas the `-rpath-link' option is only
effective at link time.
3. On an ELF system, if the `-rpath' and `rpath-link' options
were not used, search the contents of the environment variable
`LD_RUN_PATH'.
4. On SunOS, if the `-rpath' option was not used, search any
directories specified using `-L' options.
5. For a native linker, the contents of the environment variable
`LD_LIBRARY_PATH'.
6. The default directories, normally `/lib' and `/usr/lib'.
If the required shared library is not found, the linker will issue
a warning and continue with the link.
`-shared'
`-Bshareable'
Create a shared library. This is currently only supported on ELF,
XCOFF and SunOS platforms. On SunOS, the linker will
automatically create a shared library if the `-e' option is not
used and there are undefined symbols in the link.
`--sort-common'
This option tells `ld' to sort the common symbols by size when it
places them in the appropriate output sections. First come all
the one byte symbols, then all the two bytes, then all the four
bytes, and then everything else. This is to prevent gaps between
symbols due to alignment constraints.
`--split-by-file'
Similar to `--split-by-reloc' but creates a new output section for
each input file.
`--split-by-reloc COUNT'
Trys to creates extra sections in the output file so that no single
output section in the file contains more than COUNT relocations.
This is useful when generating huge relocatable for downloading
into certain real time kernels with the COFF object file format;
since COFF cannot represent more than 65535 relocations in a
single section. Note that this will fail to work with object file
formats which do not support arbitrary sections. The linker will
not split up individual input sections for redistribution, so if a
single input section contains more than COUNT relocations one
output section will contain that many relocations.
`--stats'
Compute and display statistics about the operation of the linker,
such as execution time and memory usage.
`--traditional-format'
For some targets, the output of `ld' is different in some ways from
the output of some existing linker. This switch requests `ld' to
use the traditional format instead.
For example, on SunOS, `ld' combines duplicate entries in the
symbol string table. This can reduce the size of an output file
with full debugging information by over 30 percent.
Unfortunately, the SunOS `dbx' program can not read the resulting
program (`gdb' has no trouble). The `--traditional-format' switch
tells `ld' to not combine duplicate entries.
`-Tbss ORG'
`-Tdata ORG'
`-Ttext ORG'
Use ORG as the starting address for--respectively--the `bss',
`data', or the `text' segment of the output file. ORG must be a
single hexadecimal integer; for compatibility with other linkers,
you may omit the leading `0x' usually associated with hexadecimal
values.
`-Ur'
For anything other than C++ programs, this option is equivalent to
`-r': it generates relocatable output--i.e., an output file that
can in turn serve as input to `ld'. When linking C++ programs,
`-Ur' *does* resolve references to constructors, unlike `-r'. It
does not work to use `-Ur' on files that were themselves linked
with `-Ur'; once the constructor table has been built, it cannot
be added to. Use `-Ur' only for the last partial link, and `-r'
for the others.
`--verbose'
Display the version number for `ld' and list the linker emulations
supported. Display which input files can and cannot be opened.
Display the linker script if using a default builtin script.
`--version-script=VERSION-SCRIPTFILE'
Specify the name of a version script to the linker. This is
typically used when creating shared libraries to specify
additional information about the version heirarchy for the library
being created. This option is only meaningful on ELF platforms
which support shared libraries. *Note Version Script::.
`--warn-common'
Warn when a common symbol is combined with another common symbol
or with a symbol definition. Unix linkers allow this somewhat
sloppy practice, but linkers on some other operating systems do
not. This option allows you to find potential problems from
combining global symbols. Unfortunately, some C libraries use
this practice, so you may get some warnings about symbols in the
libraries as well as in your programs.
There are three kinds of global symbols, illustrated here by C
examples:
`int i = 1;'
A definition, which goes in the initialized data section of
the output file.
`extern int i;'
An undefined reference, which does not allocate space. There
must be either a definition or a common symbol for the
variable somewhere.
`int i;'
A common symbol. If there are only (one or more) common
symbols for a variable, it goes in the uninitialized data
area of the output file. The linker merges multiple common
symbols for the same variable into a single symbol. If they
are of different sizes, it picks the largest size. The
linker turns a common symbol into a declaration, if there is
a definition of the same variable.
The `--warn-common' option can produce five kinds of warnings.
Each warning consists of a pair of lines: the first describes the
symbol just encountered, and the second describes the previous
symbol encountered with the same name. One or both of the two
symbols will be a common symbol.
1. Turning a common symbol into a reference, because there is
already a definition for the symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by definition
FILE(SECTION): warning: defined here
2. Turning a common symbol into a reference, because a later
definition for the symbol is encountered. This is the same
as the previous case, except that the symbols are encountered
in a different order.
FILE(SECTION): warning: definition of `SYMBOL'
overriding common
FILE(SECTION): warning: common is here
3. Merging a common symbol with a previous same-sized common
symbol.
FILE(SECTION): warning: multiple common
of `SYMBOL'
FILE(SECTION): warning: previous common is here
4. Merging a common symbol with a previous larger common symbol.
FILE(SECTION): warning: common of `SYMBOL'
overridden by larger common
FILE(SECTION): warning: larger common is here
5. Merging a common symbol with a previous smaller common
symbol. This is the same as the previous case, except that
the symbols are encountered in a different order.
FILE(SECTION): warning: common of `SYMBOL'
overriding smaller common
FILE(SECTION): warning: smaller common is here
`--warn-constructors'
Warn if any global constructors are used. This is only useful for
a few object file formats. For formats like COFF or ELF, the
linker can not detect the use of global constructors.
`--warn-multiple-gp'
Warn if multiple global pointer values are required in the output
file. This is only meaningful for certain processors, such as the
Alpha. Specifically, some processors put large-valued constants
in a special section. A special register (the global pointer)
points into the middle of this section, so that constants can be
loaded efficiently via a base-register relative addressing mode.
Since the offset in base-register relative mode is fixed and
relatively small (e.g., 16 bits), this limits the maximum size of
the constant pool. Thus, in large programs, it is often necessary
to use multiple global pointer values in order to be able to
address all possible constants. This option causes a warning to
be issued whenever this case occurs.
`--warn-once'
Only warn once for each undefined symbol, rather than once per
module which refers to it.
`--warn-section-align'
Warn if the address of an output section is changed because of
alignment. Typically, the alignment will be set by an input
section. The address will only be changed if it not explicitly
specified; that is, if the `SECTIONS' command does not specify a
start address for the section (*note SECTIONS::.).
`--whole-archive'
For each archive mentioned on the command line after the
`--whole-archive' option, include every object file in the archive
in the link, rather than searching the archive for the required
object files. This is normally used to turn an archive file into
a shared library, forcing every object to be included in the
resulting shared library. This option may be used more than once.
`--wrap SYMBOL'
Use a wrapper function for SYMBOL. Any undefined reference to
SYMBOL will be resolved to `__wrap_SYMBOL'. Any undefined
reference to `__real_SYMBOL' will be resolved to SYMBOL.
This can be used to provide a wrapper for a system function. The
wrapper function should be called `__wrap_SYMBOL'. If it wishes
to call the system function, it should call `__real_SYMBOL'.
Here is a trivial example:
void *
__wrap_malloc (int c)
{
printf ("malloc called with %ld\n", c);
return __real_malloc (c);
}
If you link other code with this file using `--wrap malloc', then
all calls to `malloc' will call the function `__wrap_malloc'
instead. The call to `__real_malloc' in `__wrap_malloc' will call
the real `malloc' function.
You may wish to provide a `__real_malloc' function as well, so that
links without the `--wrap' option will succeed. If you do this,
you should not put the definition of `__real_malloc' in the same
file as `__wrap_malloc'; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to `malloc'.

File: ld.info, Node: Environment, Prev: Options, Up: Invocation
Environment Variables
=====================
You can change the behavior of `ld' with the environment variable
`GNUTARGET'.
`GNUTARGET' determines the input-file object format if you don't use
`-b' (or its synonym `--format'). Its value should be one of the BFD
names for an input format (*note BFD::.). If there is no `GNUTARGET'
in the environment, `ld' uses the natural format of the target. If
`GNUTARGET' is set to `default' then BFD attempts to discover the input
format by examining binary input files; this method often succeeds, but
there are potential ambiguities, since there is no method of ensuring
that the magic number used to specify object-file formats is unique.
However, the configuration procedure for BFD on each system places the
conventional format for that system first in the search-list, so
ambiguities are resolved in favor of convention.

File: ld.info, Node: Commands, Next: Machine Dependent, Prev: Invocation, Up: Top
Command Language
****************
The command language provides explicit control over the link process,
allowing complete specification of the mapping between the linker's
input files and its output. It controls:
* input files
* file formats
* output file layout
* addresses of sections
* placement of common blocks
You may supply a command file (also known as a linker script) to the
linker either explicitly through the `-T' option, or implicitly as an
ordinary file. Normally you should use the `-T' option. An implicit
linker script should only be used when you want to augment, rather than
replace, the default linker script; typically an implicit linker script
would consist only of `INPUT' or `GROUP' commands.
If the linker opens a file which it cannot recognize as a supported
object or archive format, nor as a linker script, it reports an error.
* Menu:
* Scripts:: Linker Scripts
* Expressions:: Expressions
* MEMORY:: MEMORY Command
* SECTIONS:: SECTIONS Command
* PHDRS:: PHDRS Command
* Entry Point:: The Entry Point
* Version Script:: Version Script
* Option Commands:: Option Commands

File: ld.info, Node: Scripts, Next: Expressions, Up: Commands
Linker Scripts
==============
The `ld' command language is a collection of statements; some are
simple keywords setting a particular option, some are used to select and
group input files or name output files; and two statement types have a
fundamental and pervasive impact on the linking process.
The most fundamental command of the `ld' command language is the
`SECTIONS' command (*note SECTIONS::.). Every meaningful command
script must have a `SECTIONS' command: it specifies a "picture" of the
output file's layout, in varying degrees of detail. No other command
is required in all cases.
The `MEMORY' command complements `SECTIONS' by describing the
available memory in the target architecture. This command is optional;
if you don't use a `MEMORY' command, `ld' assumes sufficient memory is
available in a contiguous block for all output. *Note MEMORY::.
You may include comments in linker scripts just as in C: delimited
by `/*' and `*/'. As in C, comments are syntactically equivalent to
whitespace.

File: ld.info, Node: Expressions, Next: MEMORY, Prev: Scripts, Up: Commands
Expressions
===========
Many useful commands involve arithmetic expressions. The syntax for
expressions in the command language is identical to that of C
expressions, with the following features:
* All expressions evaluated as integers and are of "long" or
"unsigned long" type.
* All constants are integers.
* All of the C arithmetic operators are provided.
* You may reference, define, and create global variables.
* You may call special purpose built-in functions.
* Menu:
* Integers:: Integers
* Symbols:: Symbol Names
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
* Assignment:: Assignment: Defining Symbols
* Arithmetic Functions:: Built-In Functions
* Semicolons:: Semicolon Usage

File: ld.info, Node: Integers, Next: Symbols, Up: Expressions
Integers
--------
An octal integer is `0' followed by zero or more of the octal digits
(`01234567').
_as_octal = 0157255;
A decimal integer starts with a non-zero digit followed by zero or
more digits (`0123456789').
_as_decimal = 57005;
A hexadecimal integer is `0x' or `0X' followed by one or more
hexadecimal digits chosen from `0123456789abcdefABCDEF'.
_as_hex = 0xdead;
To write a negative integer, use the prefix operator `-' (*note
Operators::.).
_as_neg = -57005;
Additionally the suffixes `K' and `M' may be used to scale a
constant by `1024' or `1024*1024' respectively. For example, the
following all refer to the same quantity:
_fourk_1 = 4K;
_fourk_2 = 4096;
_fourk_3 = 0x1000;

File: ld.info, Node: Symbols, Next: Location Counter, Prev: Integers, Up: Expressions
Symbol Names
------------
Unless quoted, symbol names start with a letter, underscore, or point
and may include any letters, underscores, digits, points, and hyphens.
Unquoted symbol names must not conflict with any keywords. You can
specify a symbol which contains odd characters or has the same name as
a keyword, by surrounding the symbol name in double quotes:
"SECTION" = 9;
"with a space" = "also with a space" + 10;
Since symbols can contain many non-alphabetic characters, it is
safest to delimit symbols with spaces. For example, `A-B' is one
symbol, whereas `A - B' is an expression involving subtraction.

File: ld.info, Node: Location Counter, Next: Operators, Prev: Symbols, Up: Expressions
The Location Counter
--------------------
The special linker variable "dot" `.' always contains the current
output location counter. Since the `.' always refers to a location in
an output section, it must always appear in an expression within a
`SECTIONS' command. The `.' symbol may appear anywhere that an ordinary
symbol is allowed in an expression, but its assignments have a side
effect. Assigning a value to the `.' symbol will cause the location
counter to be moved. This may be used to create holes in the output
section. The location counter may never be moved backwards.
SECTIONS
{
output :
{
file1(.text)
. = . + 1000;
file2(.text)
. += 1000;
file3(.text)
} = 0x1234;
}
In the previous example, `file1' is located at the beginning of the
output section, then there is a 1000 byte gap. Then `file2' appears,
also with a 1000 byte gap following before `file3' is loaded. The
notation `= 0x1234' specifies what data to write in the gaps (*note
Section Options::.).

File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions
Operators
---------
The linker recognizes the standard C set of arithmetic operators,
with the standard bindings and precedence levels:
precedence associativity Operators Notes
(highest)
1 left ! - ~ (1)
2 left * / %
3 left + -
4 left >> <<
5 left == != > < <= >=
6 left &
7 left |
8 left &&
9 left ||
10 right ? :
11 right &= += -= *= /= (2)
(lowest)
Notes: (1) Prefix operators (2) *Note Assignment::.

File: ld.info, Node: Evaluation, Next: Assignment, Prev: Operators, Up: Expressions
Evaluation
----------
The linker uses "lazy evaluation" for expressions; it only calculates
an expression when absolutely necessary. The linker needs the value of
the start address, and the lengths of memory regions, in order to do any
linking at all; these values are computed as soon as possible when the
linker reads in the command file. However, other values (such as symbol
values) are not known or needed until after storage allocation. Such
values are evaluated later, when other information (such as the sizes of
output sections) is available for use in the symbol assignment
expression.